Correct Answer - C
For an atom with a single electron, bohr atom model is applicable.
As the value of attraction between a proton and electron is proportional to `e^2`, for an ion with a single electron, `(e^2)/(4piepsilon_0)` is replaced by `(Ze^2)/(4piepsilon_0)` i.e. `r_(n)prop(n^2)/(Z)`.